Prior to our work, abatacept had not been tested for its ability to prevent GvHD in BMT patients. However, there was considerable evidence from murine models to suggest that it might be an active compound against the immune activation that occurs during GvHD.29-33 In addition, our research group developed a non-human primate model of GvHD 34 and used this model to demonstrate that an abatacept-containing immunosuppressive regimen could significantly protect against the development of primate GvHD. 34 These results, along with the clinical evidence for efficacy of abatacept and belatacept in both autoimmunity and solid organ transplantation provided the rationale for the development of a first-in-disease feasibility trial of abatacept for GvHD prevention (Clinical Trials.org #NCT01012492). This trial, which has now completed enrollment, has documented encouraging early results with respect to both the safety and efficacy of abatacept for GvHD prevention (Kean et al., ASH 2011). These results have led to the creation of the current Phase II clinical trial of abatacept for prevention of severe aGvHD.